Wilhelm Fliess (1858–1928) was a German otolaryngologist who practiced in Berlin. On Josef Breuer's suggestion, he attended several conferences of Sigmund Freud in 1887 in Vienna, and the two soon formed a strong friendship. Through their extensive correspondences and a series of personal meetings ("congresses" as Freud described them), Fliess came to play an important part in the development of psychoanalysis.

Fliess developed several idiosyncratic theories, such as reflex nasal neuroses, postulating a connection between the nose and the genitals, and vital periodicity, forerunner of the popular concepts of biorhythms that never found scientific favor outside of psychoanalytic circles, though others, such as the idea of innate bisexuality, were incorporated into Freud's theories. Freud referred occasional patients to him for treatment of their neurosis through anaesthetization of the nasal mucosa with cocaine, and through nasal surgery. Together, Fliess and Freud developed a Project for a Scientific Psychology, which was later abandoned.

Emma Eckstein (1865-1924) had a particularly disastrous experience when Freud referred the then 27 year old analysand to Fliess for surgery to remove the turbinate bone from her nose, ostensibly to cure her of premenstrual depression. Eckstein haemorrhaged profusely in the weeks following the procedure, almost to the point of death as infection set in. Freud consulted with another surgeon, who removed a piece of surgical gauze that Fliess had left behind.[1] Eckstein was left permanently disfigured, with the left side of her face caved in. Despite this, she remained on very good terms with Freud for many years, becoming a psychoanalyst herself.

Fliess also remained close friends with Freud. He even predicted Freud's death to be near the age of 51, through one of his complicated bio-numerological theories ("critical period calculations"). Their friendship, however did not last to see that prediction out: in 1904 their friendship disintegrated due to Fliess's belief that Freud had given details of a periodicity theory Fliess was developing to a plagiarist. Incidentally Freud died at 83 years of age.

Freud ordered that his correspondence with Fliess be destroyed. It is known today only because Marie Bonaparte bought their letters and refused to permit their destruction.

Interestingly Fliess's son Robert later became an eminent psychoanalyst also.